Surface mount technology in general and ball grid array technology in particular are becoming increasing popular choices for integrated circuit packaging. Both the size and pin count of surface mount components continue to increase. This trend aggravates the problem of solder joint failure due to coefficient of thermal expansion mismatch between the printed circuit board and components mounted thereon. This problem is particularly acute for ball grid array components. Therefore, although the invention is applicable to any component packaging technology, it will be discussed using a ball grid array component as an example.
A bottom view of a ball grid array (BGA) component 10 is illustrated in FIG. 1. The underside of the BGA component 10 contains a plurality of solder bumps 12. Each solder bump 12 is electrically connected to an internal lead (not shown) which itself is connected to an integrated circuit formed on a silicon wafer (also not shown) inside the BGA component.
FIG. 2 illustrates the connection of a BGA component 10 to a printed circuit board 20. Each solder bump 12 is soldered to a corresponding trace pad 28 on the printed circuit board 20. As can be seen with reference to FIG. 5, the trace pad is an enlarged portion of the trace 27. Referring back to FIG. 2, the trace pads 28 are in positions corresponding to the positions of the solder bumps 12 on the BGA component 10. The other portions of the traces 27 are narrower to allow space for traces 27 between the trace pads 28. The solder bumps 12 may be attached to the corresponding trace pads 28 by well known methods such as reflow soldering or laser soldering.
When power is applied to an integrated circuit, some of that power is converted to heat by the movement of electrons through the integrated circuit. When integrated circuits are heated, they expand. The amount of expansion relative to the heat of a material is expressed as a quantity known as the coefficient of thermal expansion. The higher the coefficient of thermal expansion, the more a material expands when it is heated.
Referring back to FIG. 1, the physical center of the EGA component 10 is indicated by the point labeled NP. The BGA component 10 is perfectly symmetrical, therefore the physical center NP of the BGA is also the thermal neutral point NP. The neutral point NP is the point on the BGA component 10 from which all thermal expansion occurs in a radial direction. Thermal expansion directions are indicated by the vectors "E" extending radially outward from the neutral point NP.
The amount of thermal expansion for each of the solder bumps 12 on the BGA component 10 is dependent upon the distance from the neutral point NP to the solder bump 12. This distance is known as the distance to neutral point, or DNP. As the DNP increases, the amount of movement of a solder bump 12 from the neutral point NP also increases. One reason that components are designed with square packages is to minimize the DNP for all connections.
Referring back to FIG. 2, the vectors "E" indicate the direction of thermal expansion of the BGA component 10 from the neutral point NP. It should be appreciated that as heat is transferred from the BGA component 10 to the printed circuit board 20, the printed circuit board 20 also expands. However, because the printed circuit board 20 and the BGA component 10 are usually made of different materials, the corresponding coefficients of thermal expansion (CTE) may also be different, or mismatched. The result is a net force (which may be positive or negative, depending upon the respective CTEs of the BGA component and the printed circuit board and the amount of heat transferred to the printed circuit board by the BGA component) in the direction of the vector E on the solder bumps 12 and trace pads 28.
When leaded components (e.g. components with `J` leads or gull wing leads) are used, the leads act as compliant members, allowing for forces on solder joints caused by coefficient of thermal expansion mismatch. However, when leadless components such as BGAs are used, the solder bump 12 is the only available compliant member.
When component sizes and corresponding DNPs are small, the forces on the solder bumps 12 are also small and do not cause a problem. However, when component sizes and corresponding DNPs are large, the forces on the solder bumps 12 are also large and can lead to failure of the solder joint. FIG. 3 is an enlarged view of a single solder bump 12 that has failed. The force on the solder bump 12 in the direction of the vector "E" caused by the CTE mismatch between the BGA component 10 and the printed circuit board 20 has caused cracks 14, 16 in the solder bump 12. A crack 14 has completely broken the connection between the solder bump 12 and the BGA component 10, resulting in an open circuit. A second crack 16 near the bottom of the solder bump 12 has also begun.
FIG. 4 illustrates one attempted solution to this problem. The spaces underneath the BGA component 10 between the solder bumps 12 and printed circuit board 20 are filled with an underfill material 18. The underfill 18 acts as an adhesive between the BGA component 10 and the printed circuit board 20 such that movement between them is prevented.
There are two main disadvantages to this solution. First, the adhesive eventually fails, leading to solder joint failure after repeated thermal cycling. Second, the underfill must be "wicked" under the BGA component 10 between the solder bumps 12, which is a time-consuming, and therefore expensive, procedure.
A second solution to the problem is to minimize the CTE mismatch between the BGA component 10 and the printed circuit board 12. The disadvantage to this solution is that the materials needed to achieve a good CTE match result in increased production costs.
What is needed is an inexpensive and reliable apparatus and method for attaching components and printed circuit boards with mismatched coefficients of thermal expansion.